The invention relates to a power screwdriver with a drive unit, a sensing device for the actual torque as well as optionally for the actual screw clamping angle, and an electronic control of the revolutions per minute and/or the torque and/or the screw clamping angle.
In German patent 43 07 131 a power screwdriver is disclosed having a drive unit having a with planetary gear downstream thereof, a torque detection at the drive shaft, an evaluation electronic circuit, and a switch-off device for the drive unit, a sensing device for detecting the rotational position of the drive unit whereby the switch-off device has a adjustable preset device for a certain torque determined by the torque detection device, and additionally an adjustable preset device, for a certain follower path determined by the rotational position detection device and wherein switching off of the drive unit after reaching the preset torque is carried out followed by a further rotation by an adjusted follower path.
Before performing a tightening operation, the required torque as well as optionally the required follower path are preset by the corresponding adjusting elements. Subsequently, the power screwdriver is placed onto the screw element and a support leg is moved against a fixed abutment. After starting the drive unit, the tightening operation is performed and the screw element is tightened so that the torque increases. The measuring-technologically determined torque value is compared to the torque value that is preset, and, when both values are identical, an internal switching signal is provided for the follower control. Based on the actual rotational position the drive unit is then rotated further by the preset follower path. After completion of the preset follower path, the switching signal is released by the switch-off device for switching off the drive unit and thus terminating the threading operation.
In the known power screwdriver the operator must determine with respect to the screw size and the material quality the required nominal torque with the aid of tables and must input the data into the electronic input device. The follower path must also be determined by the operator and optionally entered into the electronic input device. The function of the follower path and how to determine it is not disclosed in the German patent 43 07 131. Also, the measuring-technologically detected torque value depends greatly on external circumstances such as thread friction and is no precise measure for the actually reached screw pretension.
The invention has the primary object to simplify the operation of a power screwdriver of the aforementioned kind and to eliminate operational errors, without requiring special knowledge by the operator. Furthermore, damage to the screws to be tightened and to the power screwdriver are to be prevented, the precision with which the tightening actions of the screws is effected is to be increased, the effects of the rotational energy of the drive unit during switch-off onto the tightening of the screw is to be reduced or eliminated, and a safe transmission of the measuring and control signals is to be ensured.